A. Field of the Invention
The field of the invention generally relates to the use of antistatic polymeric materials, especially polyurethane foams, to package, store, etc., electrostatic sensitive apparati, devices, etc.
B. Prior Art
Accumulation of electrostatic charges on polymeric materials has been a long standing problem when using such materials in and around electrostatic sensitive environments or devices. One type of polymeric material which undergoes buildup of static charges is polyurethane foam.
For instance, polyurethane foams have been used in environments where electronic components or devices may be subject to a static charge. In some applications the foams are used to cushion and protect packaged electronic components or devices, wherein troublesome charges are usually either generated by people handling the devices or by the environment itself. Typically, charges will accumulate on the foam during shipping, transporting, or otherwise moving the electronic components from place to place. Moreover, the synthetic materials which are used for floor covering, as well as wearing apparel for production workers, are subject to rapid static-charge buildup. Likewise, other activity during the normal course of business in shipping electronic components and devices may generate static charges. See U.S. Pat. Nos. 4,231,901 and 4,301,040 to Berbeco.
In environments described above, 100 volts (V) of static charge can be easily generated and it has been found that some metal oxide semiconductor (MOS) devices can be destroyed by less than 100V of static charge. Field-effect transitors (FET) are also damaged by such charges. Further, because personnel handling such devices can easily hold over 10,000V of static charges, MOS manufacturers have been forced to utilize additional equipment and safeguard systems to prevent static-charge accumulation. For instance, MOS devices are often packaged in conductive materials, or with spring clip and conductive dual in-line sticks. These forms of packaging short or shunt the devices, so that all have the same electrical potential. Some manufacturers ship their devices with conductive leads shorted with a wire spring clip or aluminum dip stick. Such protection is also used for MOS/FET circuits, as well as microprocessor chips and various integrated circuits. See U.S. Pat. No. 4,231,901 to Berbeco.
In conjunction with the use of the safeguard systems described above, there have also been developments to provide antistatic packaging materials, e.g. foams, which are economical and easily manufactured. Acceptable antistatic standards for a variety of materials which are used in conjunction with sensitive electronic components and devices are set forth in the U.S. Military Specification MIL-B-81705B.
To impart antistatic properties to foams, conventional antistatic additives, such as carbon black, have been used. See U.S. Pat. No. 4,231,901 to Berbeco. In one manner, the additives are incorporated in the foam by coating the additives onto the foam with a binder mixture. Besides carbon black, finely-divided particles of alumina-silicate and silver, aluminum and metal salts have also been used. Graphite fibers are also known to impart antistatic properties. These particles and/or fibers have been used in some foam layers in amounts ranging from 2 to 40% by weight of the conductive layer. See U.S. Pat. No. 4,301,040 to Berbeco.
Another prior art method of imparting antistatic properties to foams comprises post treatment of the foam so that the antistatic additives interpenetrate the foam's structure. For instance, U.S. Pat. No. 4,578,406 to Volz discloses post-treating a foam with swelling agents to expand the foam's physical structure, introducing antistatic additives through expanded cells and then shrinking the foam to trap the additive in the foam's structure. The additives used by Volz are quaternary ammonium compounds such as hydroxy alkylene quaternary ammonium salts.
Another additive used to impart antistatic properties, i.e. conductivity, to foams, is a metal ionizable salt, such as sodium thiocyanate, in conjunction with an enhancer such as a phosphate ester. Foams containing these additives have been made from high molecular weight polyether polymers. Salts such as sodium tetraorganoborides have also been used with an enhancer to impart satisfactory antistatic properties. See U.S. Pat. No. 4,617,325 to Knobel et al.
However, all of the above attempts to impart antistatic properties incur problems. For example, the equipment used to protect MOS devices and FETs incurs extra costs. Post treatment of foams to impregnate the foam with antistatic additives also involves extra costs because of the additional processing used. Further, coating foams with carbon black leads to contamination problems because these additives tend to slough off during use. Antistatic additives such as Volz's quaternary ammonium compounds also tend to slough off foams and lead to contamination as well as result in non-permanent antistatic properties. See '325 patent to Knobel. See U.S. Pat. No. 4,605,684 to Pcolinsky, Jr.